The invention relates to nozzles for spraying fuel into gas turbine combustion chambers and in particular to an improvement of the airblast-type nozzle.
Combustion chambers of gas turbines conventionally include a metal shell or liner which defines a volume in which combustion takes place. Space is limited and it therefore is important that combustion take place as quickly and uniformly as possible. This requires not only fine atomization of the fuel being injected but a uniform distribution thereof.
A conventional fuel pressure atomizing nozzle distributes and atomizes the fuel adequately at part power ratings. As load is increased on the turbine, however, the increased fuel flow leads to very high pressure drop across the nozzle and very fine droplets producing poor penetration and distribution of the fuel in the combustor.
Accordingly, airblast type spray nozzles have been introduced. Such nozzles generally use the airflow for the source of atomizing and distribution energy since the airflow patterns tend to stay relatively constant as load is increased.
Conventionally such nozzles would include a central primary flow of air inside an annular zone in which fuel is introduced. Surrounding the fuel is an annular introduction of secondary air, with tertiary air occasionally directed from a location slightly more remote from the fuel. Additional dilution air is introduced downstream of the combustion process to limit the temperature entering the gas turbine to an acceptable limit.
U.S. Pat. No. 3,713,588 illustrates such a nozzle wherein the fuel is introduced outwardly through a series of orifices into the secondary air stream. This swirling secondary air stream provides the atomizing force and energy to disperse the fuel. In accordance with the teachings of that patent the primary centrally located air is introduced for the purpose of providing an ample supply of air to the interior of the fuel spray cone. A set of helical swirler vanes are illustrated and it is stated that the interior air may be introduced without any swirl at all.
Specific relative locations are shown between the vanes swirling the secondary air and the orifices for the entrance of fuel. The objective in the teaching of that patent is to obtain concentrations of air at the location of the orifices.
Another airblast injector is known wherein the fuel is swirled for the purpose of filling an annular space from which it passes out at a relatively low velocity. The swirl of primary air is used to disperse and atomize the fuel as it exits the fuel nozzle. The swirl of airflow has been obtained by the use of helical vanes.
Helical vanes are simpler and less expensive to form than cambered vanes. Cambered vanes, however, have been used on secondary airflow where the major portion of combustion supporting air is supplied and there is a need to pass a substantial amount of air through a limited space. In such case the lower pressure drop characteristic of the cambered vanes was sufficient to justify the additional expense of their manufacture. The primary air vane swirler is very small with an outside diameter on the order of one-half inch. The need has not been to supply a large quantity of air through a small space but only to obtain a swirl. Accordingly, conventional wisdom has not suggested anything other than the more easily manufactured, less expensive helical swirler which has always been used at this location.